Making light from 'useless' heat

UvA researchers show that silicon nanocrystals can convert thermal energy into light

10 October 2017

Heat is generally considered the most 'useless' form of energy. Light, on the other hand, is a very useful form: it can easily transport energy over large distances and without loss. An international team of researchers, among which several from the UvA's Institute of Physics, now show that heat can sometimes be transformed into light. Their results were published in the latest issue of Light: Science & Applications.

Heat is the most chaotic and the most difficult to control form of energy. On the other hand, light is the most 'elegant' one: a light beam rapidly transports identical energy quanta over unlimited distances without any loss whatsoever. This is in stark contrast to, for example, the transport of electricity, another convenient form of energy used by society. Once it arrives at its destination, energy carried by light can easily be converted into electricity (solar cells), chemical energy (water splitting, light-driven catalysis) or heat.

From heat to light

In their most recent paper, appearing in the Nature-Springer Publication Group journal Light: Science & Applications, IoP-researchers and their collaborators demonstrate that tiny grains of crystalline silicon – Si nanocrystals – can do the seemingly impossible: convert part of their thermal energy (phonons), into light (photons). This research was carried out by Elinore de Jong, a recently graduated PhD student from the TGG work group of Tom Gregorkiewicz at the Van der Waals-Zeeman Institute of IoP, working together with her MSc student Huub Rutjes and flanked by postdoc Antonio Capretti and a team of international collaborators from Columbia University (New York), Ioffe Institute (St. Petersburg) and Charles University (Prague).

Turning heat into light? Image: Huub Rutjes.

Increased efficiency

Heat dissipation (phonon generation) leads to unwanted lower efficiency in many applications. In particular, in solar cells a large part of the energy of photons from the 'blue' side of the solar spectrum is lost to heat. In their research effort, De Jong and her colleagues addressed this problem by going into the relatively unexplored field of phonon management in nanostructures. For this purpose, they used layers of silicon nanocrystals (with a size of roughly a nanometer) dispersed in a silicon dioxide matrix. In their paper they show that the relatively high concentration of phonons due to the confinement in the nanostructure increases the nanocrystals’ radiative emission rate, and therefore leads to a more efficient light generation.

These results are the first steps towards purposeful manipulation of heat in nanostructures. Since the researchers have found this effect in the non-toxic and abundantly available material Si, which has already found its applications in many fields, these results are directly relevant for applications such as Si photonics and photovoltaics.